This invention relates to cables in general and more particularly to a method and apparatus for applying a water repellent substance into the cable core of a multi-pair cable, especially a communication cable.
Methods are known for applying a water repellent substance into the cable core of a multi-pair cable where the cable core is led through a filling vessel consisting of an inlet stub, a filling head and an outlet stub, and in which the water repellent substance to be applied into the cable core is contained at elevated temperature and relatively high pressure.
With the transition from paper-insulated to plastic-insulated cable conductors, especially in multi-pair communication cables, the problem of leakage water spreading over large sections of the cable has arisen. Such leakage can destroy the cable to a large extent or make it inoperative. While the formerly used paper insulation formed a natural obstacle to the spreading of the leakage water by swelling when water is present, this obstacle no longer exists when plastic insulation is used. Ever since plastic-insulated cable conductors have been adopted, therefore, there have been attempts to build up special batteries for leakage water that may get in.
In the meantime, the literature related to this problem has become very extensive. The remedy best suited for this problem has been found to be filling the voids between the cable conductors with a water repellent substance of high viscosity. Accordingly, a large number of proposals exist as to the form in which such substances can be applied to a cable. The question as to in what way it can be ensured that all voids are filled with this substance is particularly discussed. If only a single void extending over greath lengths is not filled, the danger of leakage water spreading along the cable remains.
In the majority of the known proposals, the procedure is to bring the water repellent substance into the core before the sheath is applied. Then, the substance is usually heated above the ambient temperature in order to reduce the viscosity substantially. Also, in order to fill all voids between the cable conductors, one works, almost without exception, with a pressure difference. In a number of cases, the cable core and the filling station are evacuated first; in many cases one works with overpressure which is to press the water repellent substance into the voids between the individual cable conductors.
In the known embodiments, that is, where overpressure as well as underpressure is used, sealing problems must be overcome. If tight seals are used in the vicinity of the entry and exit points of the cable core into or out of the filling station, there is danger that the cable core might be damaged by excessive mechanical stress. If, on the other hand, the seal is not sufficient, adequate vacuum cannot be maintained in the filling station or, in the case of overpressure, the necessary pressure of the water repellent cannot be obtained.